The present invention relates to the production of chlorosilanes and more particularly to a process for recovering elementary silicon from residual silicon powder and recycling pure silicon.
The present commercial method for manufacturing organohalosilanes is well known and is described in U.S. Pat. No. 2,380,995--Rochow. Rochow discloses the direct reaction of an organo-halide such as methylchloride with silicon particles in order to produce organochlorosilane. Intermixed with such particles of silicon are particles of copper, thereby forming a reactive mass. In commercial practice this reaction is generally carried out in one of three types of equipment: the stirred bed type of reactor as described in Sellers U.S. Pat. No.2,449,821, the fluidized bed reactor described in Reed, et al U.S. Pat. No. 2,389,931, or the rotary kiln.
Organotrichlorosilanes and diorganodichlorosilanes are the two basic products of the above described direct process reaction. Such compounds are utilized in the production of organopolysiloxane resins as described in U.S. Pat. Nos. 2,258,218 thru 2,258,222. Other products include organopolysiloxane fluids as described in U.S. Pat. No. 2,469,888 and U.S. Pat. No. 2,469,890 as well as the organopolysiloxane elastomers described in U.S. Pat. No. 2,448,756. These patents are generally considered to be the pioneers in the polysiloxane area. Since that time the silicone industry has experienced substantial innovation in this field and a substantial patent literature has evolved relating to the different types of compositions that can be produced from basic organochlorosilanes. It is preferable to produce the diorganodichlorosilanes in a high production manner since they can be utilized most widely, particularly in producing the linear polysiloxane fluids and polymers used in the production of heat cured rubber elastomers and room temperature vulcanizable silicone rubber compositions of various types. Along with these organochlorosilanes there are produced minor amounts of other organochlorosilanes, however, these are not as important as the diorganodichlorosilanes. It is preferable to keep the production of organotrichlorosilanes to a minimum in this process. Since organotrichlorosilanes only produce branch-chained fluids and certain resins, they are fundamentally less useful than the diorganodichlorosilanes discussed above. It is ordinarily necessary that such organotrichlorosilanes be converted to other types of organochlorosilanes before utilizaton in silicone production. Accordingly, it is preferred that in the production of organochlorosilanes by the direct process of Rochow that the weight ratio of triorganochlorosilane to diorganochlorosilane (T/D) be about 0.1 during the production of chlorosilanes by the Rochow process and preferably not exceeding approximately 0.35. However, it has been found that in most commercial manufacturing operations the ratio will be at about the 0.15 level when the reactor is started up with new material but after a period it will rise to an excess of the 0.2 level. An early solution to this problem was to remove the old silicon particles and the copper catalyst in the reactor and insert new particles whereupon the reaction could be restarted, however, this was expensive in terms of manufacturing costs.
Much research has been directed to finding a method for more fully utilizing the silicon particles in the reactor such that the weight ratio of the organotrichlorosilanes (known as T) to diorganodichlorosilanes (known as D) could be maintained at a desired level for a longer period of time thereby resulting in the maximum utilization of the silicon particles to produce diorganodichlorosilanes. One of the breakthroughs in this area is U.S. Pat. No. 3,133,109--Dotson which is hereby incorporated by reference. Dotson disclosed that the silicon particles could be more fully utilized and the amount of diorganodichlorosilane could be maximized by passing used particles from a fluid bed reactor through an external fluid energy mill. As an alternative to the external fluid energy mill, Dotson also disclosed the passing of the used silicon particles that were recycled from the reactor through a plurality of sonic jets located at the base of the reactor to create a comminution of the particles or the breaking up of the silicon particles as a result of the particles striking each other or the walls of the reactor.
It was found that by utilizing the Dotson method there could be obtained from the same amount of silicon particles a larger amount of diorganodichlorosilane such that the ratio could be kept near the desired 0.15 level and would remain less than the 0.35 level for a longer period of time. However it has also been found that the Dotson process causes under utilization of approximately 12 to 15 percent of the silicon which was introduced into the reactor and which must be removed as waste silicon from the process. It was generally considered that such silicon was poisoned and therefore no longer capable of being utilized to produce diorganodichlorosilanes.
Accordingly it was highly unexpected that the fine particles located in the fluid bed reactor of Dotson could be treated and reutilized to produce diorganodichlorosilanes such that the ratio of T to D did not exceed 0.35 for a sufficiently long period of time and such that the amounts of silicon metal lost as waste from the overall process would be diminished.
In copending U.S. application Ser. No. 132,718 filed Mar. 24, 1980 by Shade there is disclosed a process for treating silicon particles within such a silicon reactor system and thereby improving the usefulness of the silicon metal particles. This application is hereby incorporated by reference. The Shade process comprises a method of treating silicon particles having generally less than forty microns average diameter size whereupon such particles are abraded to remove the surface coating thereon and whereupon the abraded particles can be returned to the reactor for further utilization.
The present invention provides another process for recovering and recycling silicon fines in an organochlorosilane reactor system and can be considered an alternative to the Shade method or can be used in conjunction with the process described by Shade for additional recycling advantages. The process of the present invention comprises a method for classifying direct process contact mass by particle size whereby the most highly poisoned or impure silicon particles are separated from the relatively unpoisoned silicon particles and only such unpoisoned particles are recycled, thereby improving the usefulness of the silicon. Thus instead of disposing the whole mass of spent silicon fines from the direct process, only a small fraction of the spent silicon fines need be disposed at any given time.
Accordingly it is an object of the present invention to provide a process for removing impurities from residual silicon particles and hence for recycling silicon metal particles more beneficially in an organochlorosilane manufacturing process.